1. Field of the Invention
The present invention relates to apparatus for sensing rotation rates. More particularly, this invention pertains to an improved rate sensor of the vibratory or tuning fork type.
2. Description of the Prior Art
Numerous arrangements exist for measuring rotation rate about a preselected axis in inertial space. Such apparatus, commonly designated a gyroscope, forms an essential element of any inertial navigation system. Gyroscopes include, for example, complex and difficult-to-manufacture gimballed spinning rotors, strapdown sensors such as the ring laser and the still-experimental fiber optic gyroscope. All of the above-named rate sensing devices are characterized by complexity of manufacture, expense of maintenance, or both.
Another system for measuring an input rotation rate about a preselected axis is based upon the principle of the tuning fork that was developed over one hundred (100) years ago. A rate sensor based upon that principle, marketed under the trademark "GYROTRON", was developed by the Sperry Gyroscope Corporation. That device, which, as all gyroscopes of the balanced resonant sensor or tuning fork type, provides significantly greater mechanical and operational simplicity than the above-mentioned types, operates on the principle that, when a tuning fork is rotated about its central axis, it possesses an angular momentum that is equal to the product of the rotation rate and the rotational moment of inertia. The reference motion of the tines of the tuning fork changes the moment of inertia cyclically. As a result, the rotation rate must change cyclically in a complementary fashion to conserve angular momentum. Thus, the physical operation of the tuning fork-type rate sensor is similar to that of a spinning ice skater who spins faster by pulling his arms in and slows down by extending them. Consequently, in a tuning fork sensor the outward-and-inward radial vibration of the tines is converted into a rotational vibration whose magnitude is proportional to the average input rate.
Prior art gyroscopes of the vibrating resonant type have been hampered by inadequate physical designs and material instability. Recently, attempts have been made to develop a sensor of piezoelectric crystalline material such as quartz that employs this principle. By utilizing such a material, device design can be simplified insofar as the apparatus may be vibrated and output detected as a function of inherent physical properties. An example of such a system is disclosed in U.S. patent Ser. No. 4,524,619 of Juergen H. Staudte entitled "Vibratory Angular Rate Sensor System". While representing an improvement in the art, the utility of the disclosed device is inherently limited by the fact that it is an open loop system. That is, in that device, which is comprised of two pairs of tines joined to a common stem that is fixed to a frame, the coriolis-induced strain in the output tine pair caused by reaction to the vibrating driven tine pair is taken as the system output.
Among the errors to which such a device is subject, those produced by the electronic noise associated with the preamplifier (operated in either a low-impedance current mode or a high-impedance voltage mode) are most significant and limiting. This first stage of the electronic output detection process is most critical in establishing the essential random walk characteristic of the device. In order to improve random walk, signal-to-noise ratio (S/N) can be enhanced by tuning the pickup tuning fork along its output axis to a frequency near the resonant frequency of the driven tuning fork. In this way, the output signal generated at the pickoff fork is amplified by its transmissibility at the driven fork's driving frequency. Unfortunately, the enhancement of the S/N ratio that results from tuning the pickoff tines to the driven tines is "paid for" by undesirable bandwidth attenuation. For example, if the pickoff tines are tuned to the precise frequency of the driven fork, the resultant envelope of the output signal will build up linearly with time for a steady input rate. Thus, such a device will act as a rate-integrating (or displacement) gyroscope until internal damping eventually limits the signal to a steady state oscillation amplitude at the drive frequency. Since the output signal (for a given rate) varies inversely with the frequency of the input, a very poor rate sensor results. Such a hypothetical device would have negligible bandwidth but it would eventually build up a very strong signal for D.C. rate inputs.